1. Field of the Invention
This invention relates to a hybrid vehicle driven by a combination of two driving sources, such as an electric motor using electric power and an internal combustion engine using fuel such as gasoline or light oil, and more particularly to a battery charge control system for a hybrid vehicle capable of finely controlling the state-of-charge of each of numerous batteries installed on the vehicle.
2. Related Art
Motor-driven electric cars, when compared with normal engine-driven vehicles, are generally advantageous in their cleanness represented by no exhaust gas emission, i.e., no air pollution, noiselessness, and excellent drivability represented by quick response during the acceleration and the deceleration.
On the other hand, electric cars have poor cruising distances due to poor electric power capacity available from present-day batteries, and are suffering from expensive energy cost per travelling distance.
To eliminate such weakness of electric cars and enhance their advantages, there have been recently proposed various types of hybrid vehicles comprising a combination of an electric motor using electric power supplied from batteries and an internal combustion engine using fuel such as gasoline or light oil.
Among them, the hybrid vehicles are roughly divided into two types. One type is a series hybrid vehicle (SHV) which uses an electric motor as a single driving source for exclusively driving the wheels of the vehicle and uses an internal combustion engine as a source for solely rotating a generator for charging the batteries.
The other type is a parallel hybrid vehicle (PHV) which selectively or alternately, otherwise simultaneously uses an electric motor and an internal combustion engine as a driving source for driving the wheels of the vehicle in accordance with travelling conditions of the vehicle, and rotates a generator for charging the batteries.
A problem of such hybrid vehicles is that continuing charging operation against a fully charged battery may unexpectedly lower the charging efficiency of the battery as well as deteriorate the durability of the battery.
To solve this problem, Unexamined Japanese Patent Application No. HEI 5-153703, published in 1993, discloses a battery charge control system which forcibly disconnects a battery from a generator to stop the charging operation when its state-of-charge value is sufficiently large.
To obtain electric power sufficient to drive an electric motor, a number of batteries are connected in series. The fact is that the state-of-charge is not the same in each of these serially connected batteries even though these batteries are operated under the same conditions.
It is usual that charging of these plural batteries is performed by detecting a single, representative state-of-charge value and charging each battery uniformly based on the detected representative state-of-charge. Namely, the generator starts charging all the batteries by the same amount regardless of actual state-of-charge of each battery.
Accordingly, for some of batteries, such a uniform charging operation by the generator will possibly cause an excessive charge. On the contrary, for some of other batteries, this uniform charging operation may cause an excessive discharge if their state-of-charge is not sufficiently high even after the charging operation is finished.
Unexamined Japanese Patent Application No. HEI 5-219608, published in 1993, discloses a monitor system providing a plurality of sensors for detecting each state-of-charge of every single battery and displaying all of state-of-charge values of individual batteries simultaneously.
Furthermore, Unexamined Japanese Patent Application No. HEI 6-75027, published in 1994, discloses another monitor system calculating an average voltage of all batteries and making a judgement that the life is over when the number of batteries having a voltage lower than the average voltage reaches a predetermined number.